Optical capabilities for space situational awareness (SSA), and in particular for the geosynchronous Earth orbit (GEO) regime are highly desired as an augmentation of current radars. In a general sense, optical telescopes are better suited than radar systems to GEO observations due to problems of reduction in detection of reflected flux. In addition, bright daylight skies make observing dim objects challenging, because of increased system noise, camera saturation and short exposure time.
Optical telescopes in the Space Surveillance Network (SSN) used to monitor the GEO satellites operate only at night. Depending upon the sensor location, night time observations might be possible for 10 to 14 hours per 24-hour day, leaving a gap in observation capability from 10 to 14 hours per day. Daytime observations are quite important, but currently are handled mostly by a few radar systems. Current radars do not provide full global coverage and thus cannot detect many GEO objects. In addition, radar sites are expensive to operate and maintain. The radars must overcome the fact that they transmit power in a beam that must be reflected and detected upon reflection. A decrease in efficacy is proportional to the beam fading in power over twice the distance between the radar and the satellite. That is referred to as the 1/r4 problem. Optical systems experience a reduction in the detection of flux reflected from the satellite proportional to 1/r2.
At 36,000 km altitude above the Earth, GEO satellites are much farther away than low earth orbit (LEO) satellites, which are often found at altitudes between 300 and 1000 km. Since the GEO satellites are approximately 100 times farther than LEOs, GEOs will be 10,000 times fainter, all else being equal. An 8th magnitude LEO object at 360 km if placed in a GEO*/orbit would become 18th magnitude object.
In addition, the bright daylight skies make observing dim objects challenging because of increased system noise, camera saturation and short exposure times. For a sky brightness increase of 100 to 500 times, which is typical of IR observing bands in the day, noise will increase by approximately the square root, or 10 times to 25 times. This reduces system detection capability by ˜3 to 5 magnitudes. At night, exposure times of minutes are used to see very dim objects, but in the day, maximum exposure times may be less than 1 second before the camera signal is saturated to a maximum level.
GEO satellites are expensive and tend to be quite large, which makes them brighter than smaller LEO objects, thus brighter than 18th magnitude. They typically have large, steerable solar arrays, which have areas in the tens of square meters. The faces enlarged somewhat mitigate the issues of distance and daylight viewing.
In FIG. 1, the results of the European Space Agency's 2006 survey of the GEO belt are shown (from R. Jehn, H. Klinkrad, H. Krag, T. Flohrer, and R. Choc. ESAs Optical Ground Station at Tenerife. OPS-G Forum, 18 Jan. 2009, 2008). This data shows that there is a nominal distribution of brightnesses at night, which can be translated to daytime magnitudes as described in the following section. It can be seen that, if the limiting magnitude of a system is equivalent to the translated visual magnitudes in the daytime geometry and wavelength of observations is equivalent to 14th to 15th magnitude, a significant fraction of the population can be followed.